1. Field of the Invention
The present invention relates generally to an apparatus for and a method of severing and sealing thermoplastic film to make plastic bags or the like, and in particular to an apparatus and a method that utilize a combination of thermal energy and mechanical force to sever and seal the thermoplastic film.
2. Description of the Related Art
In the production of plastic bags, a continuous web of thermoplastic film typically is supplied to a bag-making machine that severs the film into segments and seals the severed edges of each segment to form a bag.
Referring to FIG. 1, a known bag-making machine is designated generally by reference numeral 10. The machine shown is a dual-lane bag-making machine. That is, two folded webs of thermoplastic film 22 are fed to the machine 10 in parallel, and the machine 10 severs and seals the parallel webs to form two sets of bags.
The bag-making machine 10 includes a continuously rotating seal drum 12 and a cam track assembly 14. Folded webs of thermoplastic film 22 are fed between the seal drum 12 and the cam track assembly 14. A plurality of clamping devices 16, such as disclosed in U.S. Pat. No. 5,964,688, assigned to the same assignee as the present invention and incorporated herein by reference in its entirety, is mounted on the cam track assembly 14 for movement therearound. As best seen in FIG. 2, each clamping device 16 comprises a pair of parallel spring-mounted seal bars 18, separated by a gap 20.
The clamping devices 16 intermittently contact the film 22 and clamp it firmly against the seal drum 12. A cam-actuated, electrically-heated hot wire 24 advances through an opening 26 in the surface of the seal drum 12, burns its way through the film 22, and moves into the gap 20 between the seal bars 18. In this way, the film 22 is severed into segments and the resulting severed edges are simultaneously sealed.
As shown in FIG. 3, a typical hot wire 24 consists of a relatively thin, elongated piece of electrically-conductive material stretched between supports 28 disposed on opposite sides of the effective cutting lengths 30 of the hot wire 24. To date, hot wires of various sizes and shapes have been used. For instance, some processes have employed hot wires having circular cross sections with diameters up to about 0.050 inches. Other processes have employed rectangular or wedge-shaped hot wires. Such conventional hot wires generally are heated to temperatures between 1000-1400° F.
Several problems exist in the production of plastic bags using conventional bag-making machines such as that described above. For example, as the hot wire burns its way through the film, a significant amount of smoke is produced. Some of this smoke condenses on components of the machine, leaving behind a wax residue. On an almost daily basis, the machine must be shut down and the components cleaned in order to remove the wax buildup. Another problem with conventional bag-making machines is that the relatively thin, high-temperature hot wires often fail due to the stress of repeated severing and sealing operations. Replacing broken wires requires additional downtime for the machine and results in a further reduction in productivity.
Accordingly, there is a need in the art for an improved apparatus for and method of severing and sealing thermoplastic film to form plastic bags or the like.
There is a further need for such an apparatus and method that reduce the amount of wax that accumulates on the components of the apparatus.
There is a still further need for such an apparatus and method wherein the hot wire used to sever and seal the film will not easily break when subjected to repeated severing and sealing operations.